A multi-center, randomized, controlled clinical trial to test the hypothesis that treatment with mycophenolate mofetil (MMF) will lead to significant and sustained improvement in proteinuria in patients with IgA Nephropathy who have been pre-treated (and continue to be treated) with ACEi and FOS compared to a placebo control group of patients receiving comparable doses of ACEi and FOS without MMF. Data for this outcome will be examined every six months and at the end of 2 years of study. Comparisons will be made between the two treatment groups for change from entry level in urine protein to creatinine (UPr/Cr) ratio, 24-hour urine protein excretion rate and estimated glomerular filtration rate (GFR).